The present invention relates to a liquid developer for electrophotography and more particularly to a liquid developer for electrophotography improved upon the reproducibility and quality of images obtained through a development and the dispersion stability of the developer.
Liquid developers are those carrying the so-called `toner` which is dispersed in a solvent, so that image forming areas may be provided in such a manner that charged toners adhere to a latent image by electrophoresis, in the course of a development process. Such toner is, therefore, comprised of a statically chargeable substance such as a pigment or dyestuff particle capable of generating the above-mentioned electrophoresis and a resin capable of being absorbed to the particle and migrating together with the particle. Such a toner particle as mentioned above is to be so stable in the charge controllability thereof as to keep its charge constant and is to be excellent in the dispersibility and disperson stability thereof to a solvent used.
The toner particle sizes involve not only the resolving power of an image but also the sharpness, flowing characteristics of an image and the like to a great extent. Generally, the liquid developers are of the type of dispering resins containing such a colorant as pigments, dyestuffs or the like and, therefore, the particle sizes thereof have a certain extent of a wide distribution. However, not so careful attentions have so far been paid to the fact that an image quality differentiates in accordance with the conditions of the particle size distribution.
With the purpose of improving the above-mentioned dispersibility, dispersion stability and fixability. Japanese Patent Examined Publication Nos. 3696/1980 and 10195/1980; and Japanese Patent O.P.I. Publication Nos. 125333/1977 and 49445/1973 disclose that, in the liquid developers for electrophotography comprising a toner whose chief ingredients are a pigment or dyestuff and thermoplastic resin and a liquid carrier in which the toner is dispersed, an alkyl acrylate or alkyl methacrylate which is soluble in a non-aqueous solvent, or a copolymer of ethylene type unsaturated monomers which is hardly soluble or insoluble in the above-mentioned non-aqueous solvents, is added as the above-mentioned resin in a proportion of the range of from 95 to 60% by weight.
They are, however, not only unsatisfactory in the characteristics originally aimed at, but also not always excellent in the sharpness and flowing characteristics of the toner images produced with the developers.
With the purpose of obtaining a toner which is so excellent in mechanical strength and printing durability of toner images and in ink receptivity as to be capable of serving as a developer when a printing plate is prepared in an electrophotographic system, Japanese Patent Application No. 113644/1983 indicates that, in a liquid developer containing a toner chiefly comprising a pigment or dyestuff and a resin, a high molecular compound comprising a monomer component which is soluble in a non-aqueous solvent and another monomer component which is hardly soluble or insoluble in the non-aqueous solvent are contained therein in a proportion of from 5 to 55% by weight. This toner is, however, poor in dispersibility and dispersion stability and is apt to cause the flow of image, because it contains a lot of waxes and/or polyolefin, say, in a proportion of from 45 to 95% by weight.
As described above, the conventional types of liquid developers, is not satisfactory in dispersion stability, can hardly provide such a toner image as is excellent in image quality and, in particular, substantially less in image flow. The improvements thereof have, therefore, been demanded so far.